1. Field of the Invention
The present invention is directed to an improved method of manufacturing ceramic capacitors of the lead filled type and to the resultant capacitors.
2. The Prior Art
It is known from U.S. Pat. Nos. such as 3,679,950; 3,879,645; and 3,965,552, by way of example, to form a ceramic capacitor which eliminates the need for using noble metals such as platinum, paladium, and the like. Prior to the method as set forth in the noted patents, ceramic capacitors were fabricated by inter-leaving green ceramic sheets with electrode areas comprised of metals capable of withstanding the sintered temperatures to which the capacitors are subjected.
In accordance with the noted patents, ceramic capacitors are formed by first forming a monolithic ceramic block having void areas exiting to end portions of the block which void areas correspond to the intended location of the capacitor electrodes. Thereafter, the void areas are filled with lead to form the electrode. Void areas are formed by introducing between the green ceramic sheets layers of organic material which are burnt out in the course of heating the green ceramic material.
Capacitors of the type described have presented significant problems in their manufacture. One such problem arises as a result of the tendency of the molten lead which is forced into the void areas to immediately run out of the voids as soon as the ceramic members are removed from the molten lead bath and depressurized.
In accordance with one proposed means of solving the lead run-out problem the ends of the ceramic monoliths were coated with a porous frit and lead is forced through the frit to fill the void areas. The difficulty with this method is that if the frit is dense and of limited porosity it is difficult to insure that the entirety of the void areas will be filled with lead with the result that the capacitance of the resultant device is unpredictable. In the event that a highly porous frit is employed, substantial lead run-out is unavoidable. Where a relatively dense frit is employed, it is necessary, in order to make contact with the buried electrodes, to grind away portions of the frit to expose the lead at the end surfaces.
In order to minimize lead run-out without resorting to covering the ends of the monolith with a dense and limited porosity frit, it has been proposed to incorporate increments of silver in the frit composition. The silver tends to prevent the lead from flowing outwardly through the frit, this tendency apparently resulting form the fact that the lead can wet to the silver. The silver frit combination does not provide a satisfactory solution to the termination problem of capacitors of the lead filled type, since the silver components are readily dissolved in the lead with a result that the silver in the frit is leached from the frit leaving external frit areas which are not electrically connected to the internal electrodes.
Attempts have been made to incorporate in the frit materials which are wet by the lead, but which are not soluble in the lead. By this means it is possible to impregnate the ceramic monoliths while eliminating the tendency of the impregnating lead to flow outwardly from the monolith. A difficulty inhering in the use of such lead wettable but non-soluble additions to the frit composition resides in the fact that increments of the lead wet not only internally to the deposited electrodes, but also adhere to the exterior of the frit. As a result of the tendency of lead to accumulate on the exterior of the termination frit, the ceramic chips tend to adhere one to the other. In order to avoid such adhesion, it is necessary to subject the impregnated monoliths to violent jogging while the same remain at a temperature above the melting point of the lead.
As will be readily recognized, jogging of a batch of impregnated chips is disadvantageous in that a substantial amount of breakage and cracking is experienced. Also, increments of lead are retained on the external surfaces of the frit of the separated chips with the result that the physical outline of the chip is largely unpredictable.
One attempted means of solving the termination problem of lead filled capacitors is set forth in U.K. application 2,199,571A, and European Patent application 0,092,912. In accordance with such applications, the ceramic capacitor bodies have their end portions covered with a frit mixture comprised of large sized ceramic particles coated with silver and glass frit. The frit mixture is fused to form porous end coatings. Thereafter the end coating are sulphided and the chips lead impregnated. The sulphiding step is said to prevent the silver from leaching out in the lead bath while still permitting the lead to wet to the silver sulphide sufficiently to prevent lead run-out. The solution proposed by the above noted patents is unsatisfactory since, by virtue of the tendency of the lead to wet to (although not dissolve) the silver sulphide, large increments of lead coat the external surfaces of the frits. It thus remains necessary to violently jog the capacitors to procure separation, and the resultant capacitors are of unpredictable dimensions as a result of excess adherent lead. It may here be observed that it is highly desirable that the exterior dimensions of ceramic capacitors be tightly controlled since, in use, such capacitors are often fed by automatic machinery and the handling of the capacitors by such machinery is in turn dependant upon the capacitors being accurately sized.
U.S. Pat. No. 4,652,967 represents a further attempt to solve the problem of terminating lead filled capacitors. In accordance with this reference a porous frit includes a paste made of Ni3B. This material is said to wet to the lead and thus retain the lead within the void areas and yet not be leached out in the course of immersion in the lead bath. Thereafter a conductive covering layer is applied over the frit areas of the impregnated ceramic. The noted reference fails, however, to avoid the problems of adhesion of the impregnated capacitors and the tendency of lead increments to stick to external surfaces of the frit with the result that violent jogging is necessary, and size variations due to adherent lead still result.